Equilibrium Definition and 1000 Threads

Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems.
In a system that is in its own state of internal thermodynamic equilibrium, no macroscopic change occurs.
Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal, mechanical, chemical, and radiative equilibria. Systems can be in one kind of mutual equilibrium, though not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by a thermodynamic operation. In a macroscopic equilibrium, perfectly or almost perfectly balanced microscopic exchanges occur; this is the physical explanation of the notion of macroscopic equilibrium.
A thermodynamic system in a state of internal thermodynamic equilibrium has a spatially uniform temperature. Its intensive properties, other than temperature, may be driven to spatial inhomogeneity by an unchanging long-range force field imposed on it by its surroundings.
In systems that are at a state of non-equilibrium there are, by contrast, net flows of matter or energy. If such changes can be triggered to occur in a system in which they are not already occurring, the system is said to be in a meta-stable equilibrium.
Though not a widely named "law," it is an axiom of thermodynamics that there exist states of thermodynamic equilibrium. The second law of thermodynamics states that when a body of material starts from an equilibrium state, in which, portions of it are held at different states by more or less permeable or impermeable partitions, and a thermodynamic operation removes or makes the partitions more permeable and it is isolated, then it spontaneously reaches its own, new state of internal thermodynamic equilibrium, and this is accompanied by an increase in the sum of the entropies of the portions.

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  1. Z

    Chemistry A calculation of equilibrium constant of a reaction of phosgene

    We start with ##n_i## moles of phosgene and zero of the products. The initial total pressure and also partial pressure of phosgene is ##\frac{n_iRT}{V}##. At equilibrium, suppose ##x## moles of phosgene have reacted. Then we have ##n_i-x## moles of phosgene left and ##x## mole of each of the...
  2. R

    Equilibrium problem: Multiple forces acting on this hinged, supported beam

    Hi, apart from the interesting wording of this question, I'm struggling to understand the concept of what we need to do here. I started by having three equations (summation Fx, summation Fy and Moment about Y). I also calculated the two reaction forces about A and the one rection force about B...
  3. T

    Engineering Problem using Pulleys with Weight in Equilibrium

    Dear all, I have the following problem: I need to find the compression "C" in the mast and the angle that the masts makes with the horizontal (X degrees). I know that the tension in the ropes "T" are of the same magnitude on both sides of the pulley, but I am struggling with finding C and X...
  4. daisy7777

    Engineering Solving for the Reaction Force by taking Moment about a Point

    For this problem, I took the moment about point B to 'get rid' of the tension force in the cables BA and BC. By doing this, I figured I could solve for the x and y components of the reaction force at D. From there, I could solve for the tension in BA and BC in the x and y and then solve for Dz...
  5. heyhey281

    B Quasistatic condition for a process involving a piston in a cylinder

    The time scale on which the change (such as a change in external parameters or a external parameters or an addition of heat) takes place is referred to as τ_exp. The relaxation time τ_relax, on the other hand, is the time that the system needs to return to a state of equilibrium after a sudden...
  6. M

    Engineering Equilibrium equations of elastic body

    Consider a structure comprising an elastic deformable body (in pink) attached to a rigid body (in yellow) at the right side ( $\Gamma_c$ interface) and fixed at the left side. Assume a force $F$ is applied to the rigid body at an angle $\beta$, as shown in the diagram. Subsequently, the...
  7. G

    I Is equilibrium vapor pressure different in vacuum and in air?

    It’s usually being assumed that points of equilibrium liquid – its vapor is given by a curve in the (P,T) coordinates, and this curve doesn’t change no matter is there another gas in the system or not. For example: if water is put in the empty volume, it will obviously vaporize, filling the...
  8. M

    How should I find the equilibrium points and the general equation?

    Consider the differential equation ## \ddot{x}+cos(x)=0 ##. Note that ## \ddot{x}=f(x, \dot{x}) ##, so we have ## f(x, y)=-cos(x) ##. Then ## f(x, 0)=-cos(x)=0 ##. This gives ## x=n\pi-\frac{\pi}{2} ## for some ## n\in\mathbb{Z} ##. Since the differential equation for the phase paths is given by...
  9. hraghav

    Find final temp of 2 substances once they have reached equilibrium

    Cooling Lerasium to its melting point: Q1 =m L⋅c L,liquid ⋅(T L,i −T L,melt) Q1 = 1.49kg⋅221.6J/kgK⋅(1108.1−56.4)K Q1 = 330.184 ⋅(1051.7) = 347254.512 J Heating Atium to its melting point: Q2 =mA⋅cA,solid ⋅(TA,melt −TA,i) Q2 = 2.44kg⋅524.8J/kgK⋅(308−33.6)K Q2 = 1280.512⋅(274.4) = 351372.493 J...
  10. l4teLearner

    To what extent does this system behave as a pendulum?

    In the formula above I have that the mechanical momentum of the horizontal force with respect to ##C## is always ##0## because the point of application coincides with the pole. Also, the mechanical momentum of the costraint reactions is ##0## because the costraint is smooth so the reaction is...
  11. M

    Classification of Equlibrium Points

    I hope this is more properly laid out? We previously established that the stationery points were (1,1) and (-1,1) For this first stage I now need to create the elements of a Jacobian maitrix using partial differentation. I am confused by reference to the chain rule. Am I correct that for dx/dt...
  12. P

    I Why does reversibility require equilibrium?

    my question is short simply why reversibility requires equilibrium?
  13. l4teLearner

    Potential of particles moving on a circle attracted by elastic force

    I use ##l-1## lagrangian coordinates ##\alpha_1,...,\alpha_{l-1}## . ##\alpha_i## is the angle between ##OP_{i-1}## and ##OP_{i}##. As the length of a chord between two rays with angle ##\alpha## is ##d=2Rsin(\alpha/2)##, I write the potential energy of the system as...
  14. Grandpa04

    Static Equilibrium Reaction Question about Fixed Point

    For this problem I set ΣFy = -3kN - 0.75kN - 3kN + Ay + By (with By representing the right side vertical force, and Ay representing the left side force). For ΣM, I used the rectangular distributed load of 0.5 kN at 3m from point A as the pivot: ΣM = -3m*Ay - (0.5kN*4m) - 3kN*6m - 5kN*m + By*6m...
  15. hraghav

    Translational Equilibrium Practice problem

  16. C

    Calculate how much weight the crane could lift

    I think the first part is F = mxa? But given the jib is resting on a turntable at a certain distance, does that mean the longer and shorted ends experience different loads? Especially when the shorter end has a 16T counterweight? The second part, I have no idea how to calculate this nor the...
  17. S

    Physics Torque Equilbrium Question

  18. brochesspro

    Pressure inside a sealed container

    I do know how to solve this question: We find the equilibrium pressure due to air molecules inside the container (after water has stopped flowing). Then, assuming initial pressure of gas inside the container to be atmospheric pressure, we use Boyle's law to find the new volume of the gas and...
  19. M

    Mass on a spring from equilibrium

    Can someone explain that, when using the formula (Fs=1/2 kx^2) why do we use x=0.1m instead of 0.05m? Seems like a simple concept but why isn't it 0.05m (since 0.05m from equilibrium). Thanks.
  20. MatinSAR

    I Question about the Zeroth law of thermodynamics and Equlibrium

    I know that if 2 systems A and B are in equilibrium their coordinates doesn't change. Systems are not complicated and be fully described using two separated coordinates X and Y. What will happen if I seprate them using an adiabatic wall? Their coordinates start to change but I cannot...
  21. Tesla In Person

    I Pulley question -- Three pulleys lifting a mass...

    Hi, I have this pulley question, i have to find the force F that needs to be applied to the string to keep the system in equilibrium. I found 150N. Is it correct? T_1= 300N and T_2=150N so F is 150N.
  22. Juanda

    The Relationship Between Masses and Angles in a Pulley System

    The problem is based on a similar thread. In fact, the first question is extremely similar. However, the second question is the one I consider more interesting but I posted the first one too for context. If this was just 1 pulley and two masses, then equilibrium is only possible if both masses...
  23. deepanshu_ch

    I Will Heating a Sealed Container Cause All Liquid to Convert to Vapor?

    Please can one explain what is exactly gonna happen if we have closed container and liquid is present in it and we heat from bottom ? Then whole liquid will converted into vapour or not ? But during the phase change both system in dynamic equilibrium. If both phases are in dynamic equilibrium...
  24. K

    Why does ##Sigma T. x=0## hold for tension

  25. putongren

    Two charged beads with a third bead in equilibrium

    Let a = distance between the charge on the left to the third bead. Since the electric field is equilibrium, we equate the electric field emanating from the left charge to the right charge at the location of the third bead. I want to try to find the ratio of d with a. E=E Kq/(d-a)^2 = K*3q/a^2...
  26. E

    Thermodynamic equilibrium for systems only open to particle exchange

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  27. G

    How to find the equilibrium position of three masses (two of them fixed)?

    Question: Suppose that two bodies of masses m1 and m2 are a fixed distance d apart, and that both of them act on a third body of mass m. Find the position of the third body such that the two forces are in equilibrium. I have solved this question and obtained the answer listed in the back of the...
  28. ogodwin

    Keeping a uniform bar in horizontal equilibrium

    I'm able to get the force of the top spring (6.75N upwards) by setting the sum of all forces equal to 0 as everything is stationary. Where I'm stuck is starting the second part of the problem. I initially tried setting the sum of torques equal to 0 using the top spring as a lever arm but unless...
  29. binbagsss

    A Deriving Non-linear acoustic wave models, equilibrium state assumption

    The standard derivation in obtaining a single wave equation involves making use of the heat equation with a Taylor expansion of the equation of state, then differentiating this equation and the continuity equation with respect to time, and combining with the divergence of the NS equation...
  30. P

    Understanding the Effect of pH on Benzoic Acid Solubility

    Suppose we put benzoic acid in a buffered solution at pH=2. In this condition, it will be almost completely unionized and therefore not very soluble. Conversely, if it is placed in a basic environment, the solubility will be high. But why does this happen? Is there a shift in the acid...
  31. Philip Koeck

    I Equilibrium between objects at different temperatures?

    I've come across a puzzling thought experiment. Consider two black bodies surrounded by vacuum. The surrounding temperature is exactly 0 K. By some ingenious optical device all the radiation from body 1 is focused onto body 2 and vice versa. If left alone sufficiently long the two bodies will...
  32. M

    Electrostatic equilibrium problem

    For this problem, I don't understand why this is impossible. Can someone please help? Many thanks!
  33. haha0p1

    Chemistry Finding equilibrium expression for an equilibrium equation

    In the d(i) part, we are asked to write an equilibrium constant. It was mentioned in the chapter that solids and liquids will not be added in the equation. So, I think that there is not going to be any Expression for this reaction as all the reactants and products are liquids. Is my answer right?
  34. haha0p1

    Equilibrium and indicator equation

    The equation for indicator is: HIn ⇌H+ + In- Which reaction (forward or reverse) will be preferred when I increase the Alkali. According to me, The reverse reaction should be preferred as the reaction will try to minimize the effect of the change. Hence In- (i.e the conjugate base) will be...
  35. haha0p1

    Finding partial pressure at equilibrium

    In the coursebook the question says: The reaction below was carried out at a pressure of 10×10⁴ Pa and at constant temperature. N2 + O2 ⇌ 2NO the partial pressures of Nitrogen and Oxygen are both 4.85×10⁴ pa  Ccalculate the partial pressure of the nitrogen(ll) oxide, NO(g) at equilibrium. In...
  36. V

    B Heat exchange after thermal equilibrium

    In screenshot below, systems A and B are separated by an adiabatic wall initially while each of them exchanges energy with system C via a diathermic wall. Once A and B reach thermal equilibrium with C, then A,B are allowed energy exchange via a diathermic wall, and energy exchange between A and...
  37. kuruman

    A Remarkable Example of Equilibrium

    During a recent visit to the Modern Museum of Art in Fort Worth I came across this remarkable sculpture. It appears to be free standing (no strings attached). I assumed it is screwed into the floor and, for obvious reasons, I didn't poke it to see if it will tip over. My thinking is that the...
  38. P

    B Understanding thermodynamic equilibrium

    My thermodynamics is rusty and my current endeavors demand me to wield it, so here we go: I need a good starting point and/or references to study physical systems in thermodynamic equilibrium. Any references which address the fluid mechanics perspective of said systems are welcome too. Let me...
  39. D

    SO2 Conversion Equilibrium Curve

    Hi... i want to draw Equilibrium curve for SO2 oxidation to SO3, i found following relations but don't know to use them... kindly tell me how to draw Equilibrium curve using these equations or any source on web from where i can get directly SO2 oxidation data Vs Temp? Equations are following...
  40. G

    Calculating ##F_{DB}##: Equilibrium in a Machine

    Figure: My attempt at a solution: For the calculation of ##F_{DB}## we consider the equilibrium in the whole machine. $$\left. \begin{array}{r} h\rightarrow h=6\cdot \sin \theta +3=6,44\, \textrm{ft} \\ d\rightarrow d=6\cos \theta -1,5=3,41\, \textrm{ft} \end{array}\right\} \,\, \alpha =\arctan...
  41. G

    Analysis of systems in equilibrium: articulated structures

    Figure: My attempt at a Solution: We calculate REACTIONS: $$\sum F_x=0\rightarrow \boxed{Bx=0}\qquad \sum F_y=0\rightarrow By+A=50$$ $$\sum M_B=0\rightarrow \boxed{A=25\, \textrm{kN}}\quad \boxed{B=25\, \textrm{kN}}$$ KNOT B: $$\alpha \rightarrow \alpha =90-40=50\, \textrm{º}$$ $$\beta...
  42. G

    Equilibrium of a rigid solid on a rod

    Figure: Attempt at a Solution: $$F=k\Delta x=lk\sin \theta$$ $$\delta x=0 \quad \textrm{when} \theta=0$$ $$\sum M_C=0\rightarrow$$ $$\rightarrow Pl\cos \theta -F_yl\cos \theta -F_xl\sin \theta =0\rightarrow$$ $$\rightarrow Pl\cos \theta -Fl\cos^2 \theta -Fl\sin^2 \theta=0\rightarrow$$...
  43. tbn032

    Chemistry Confusion in relation of Gibbs free energy and equilibrium constant

    SO2(g)+1/2O2(g)⇌SO3(g);ΔHo=-98.32KJ/mole,ΔSo=-95J/(mole-K). find Kp at 298 Kelvin? In given question at first Δ G will be calculated using formula ΔG = Δ H – T x ΔS, by putting the given values in formula we get ΔG = -70.01 kJ/mol. Then Keq will be calculated using equation = Δ G = -RT ln Keq...
  44. bbsgirl10

    Static Equilibrium of a Rod attached to a Pivot

  45. majorjc

    I Heat diffusion through grain mash to reach equilibrium

    I was reading posts this morning on another forum and came across a question that made me start thinking about physics. Since I've always tried to satisfy my curiosity through reading and trying to learn new things, or trying to remember the things that I've forgotten years ago, I went to...
  46. Bill Dreiss

    A Resolve Discrepancies in Boltzmann's Equilibrium Theory

    In Lectures on Gas Theory (Dover Books on Physics) (p. 74), Boltzmann states “In nature, the tendency of transformations is always to go from less probable to more probable states”, by which he means what are now called macrostates. Thus he claims that an ideal gas almost always evolves to the...
  47. L

    Conducting rod in equilibrium due to magnetic force

    I am having problems understanding point (b) so I would like to know if my reasoning in that part is correct and/or how to think about that part because I don't see how to justify the assumption ##v_y=0\ m/s##. Thanks. I set up the ##xyz## coordinates system in the usual way with ##xy## in the...
  48. Simobartz

    I Minimization of thermodynamic equilibrium

    Hi, I don't understand what does it mean that at equilibrium the proper thermodynamic potential of the system is minimized. For example on the book Herbert B. Callen - Thermodynamics and an Introduction to Thermostatistics it is written: Helmholtz Potential Minimum Principle. The equilibrium...
  49. Dario56

    I Concept of Thermal Equilibrium in the Context of Canonical Ensemble

    Canonical ensemble can be used to derive probability distribution for the internal energy of the closed system at constant volume ##V## and number of particles ##N## in thermal contact with the reservoir. Also, it is stated that the temperature of both system and reservoir is the same, i.e...
  50. NovatoOfPhysica

    Virtual Work and Static Equilibrium

    Hi there, this question has already been discussed in this forum, however I do not know how to proceed further and if my reasoning is correct. I start by imagining a downard displacement of the 1kg weight. As a consequence of this, block A moves upward and to the right. Also, block B moves...
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